“In The Last Jedi, Leia is seen flying in space towards the Resistance flagship, with zero build-up at all.” - she isn’t flying - she is using the Force to pull herself to the ship.
Yes, the scene wasn’t filmed / shot well at all - and has given rise to a claims of ‘Leia flies’ etc… yet no, she didn’t fly.
The “with zero build-up at all”…
Well, you just demonstrated that Leia is Force sensitive from ESB and ROTJ… yet you forgot where there is a force connection with her son in TFA as he contemplated firing on the lead command ship. So it’s not a surprise to learn she also had knowledge of, or could pull off with a certain degree, other aspects of using the Force.
Though you do not see it on-screen would it be surprising that as a child… Kylo spent time with Leia and Luke all together using the force for fun, games or education etc? Like families do?
As for Leia’s force-sensitivity “completely undermines the entire quest to find Luke in The Force Awakens”…
It is explained in TFA… that Luke cut himself off from the Force - and seemingly the ability for other force-users to be able to find him - not just his sister Leia… but more experienced, powerful and adept force-users like Kylo and Snoke.
That Leia would not be strong in the force enough able to battle Kylo or Snoke should come as no surprise - she has devoted her adult life since ROTJ to serving the Galaxy as a politician - and not practicing increasing her Force abilities to such a level to fight ‘Dark Side’ users.
TROS Spoiler:
Leia dumping Luke on his backside whilst ‘training’ in a flashback some 25 years before… doesn’t mean Leia could have gone up against Snoke or Kylo - or the Emperor. Yoda MAY have referred to her once as being ‘another’ from the ‘No, there is another…’ line of dialogue - but her life has changed considerably since then…
Luke remained their last best hope… until the Force awakened in Rey, and she became ready to stand against them (Kylo, Snoke, and now the Emperor…